Binary and ternary metal aluminum oxides are technologically important classes of materials with applications in ceramics, optics, electronics, lasers and catalysis. MgAl.sub.2 O.sub.4 (spinel) can be hot pressed into transparent windows with an exceptional infrared transmission range. MgAl.sub.2 O.sub.4 is also used as refractory material for metal refining operations. Y.sub.3 Al.sub.O.sub.12 (YAG) is widely used as a solid state laser material (Nd:YAG), and as a high energy phosphor (blue). YAG also has high temperature chemical stability and the highest creep resistance of any known oxide, leading to its evaluation as a promising fiber material for the preparation of ceramic composites. LaAlO.sub.3 is being used as a substrate material for growth of thin film oxide superconductors. Mullite, an aluminosilicate, and cordierite (magnesium aluminosilicate) have exceptional high temperature shock resistance and chemical stability and the latter material is widely used in automotive catalytic converters.
The first requirement of a successful ceramic process is the availability of a good powder. Most of the advances in ceramic processing can be traced directly to the development of powders with controlled size and purity. Early ceramic processing used mineral based precursors that had a wide range of grain sizes and impurities. Ceramics formed from the mineral precursors often had glassy phases that reduced the performance of the ceramic. The development of solution based precipitation techniques that produced fine powders with well-defined particle size distributions and high purity significantly improved the fabrication of ceramic materials.
The traditional ceramic methods that are used to prepare binary (or more complex) aluminum oxides involves the physical mixing of the powders of the oxides (or oxide precursors), sintering at high temperatures for extended reaction times, grinding and re-sintering. High temperatures and long reaction times are necessary to overcome slow solid state diffusion since physical mixing is limited to the micron scale. The traditional ceramic approach provides a simple route to the preparation of powders, however the powders must then be formed into the desired shape by traditional processing methods (slip casting, extrusion, dry pressing etc.) and sintered to fabricate the shaped ceramic item.
In order to overcome the limitations of the traditional ceramic powder processing methods, chemical routes to the synthesis of complex aluminum oxide powders and ceramics are increasingly being adopted. The most widely employed methods are the sol-gel based techniques whose versatility, and potentially atomic level homogeneity, make them desirable approaches for the preparation of arrange of materials and forms. Sol-gel synthesis of alumina has traditionally been performed by the neutralization of a concentrated aluminum salt solution. However, the strong interactions between the freshly precipitated alumina gels and ions from the precursors solutions make it difficult to prepare the gels in pure form. To avoid this complication alumina gels are often prepared from the hydrolysis of aluminum alkoxides, [Al(OR).sub.3 ].sub.n, i.e., ##STR1##
Although there can be significant advantages to the synthesis of metal aluminum oxides via the sol-gel route (such as preparation of small particles size and good homogeneity) there are a number of significant difficulties, including: long reaction times required (often greater than 24 hours), the addition of complexation agents that are necessary for inhibition of premature precipitation, careful pH control of the sol, low yields of gels from the alkoxide precursors, and the relative instability of the sols. The instability of the sols means that the sols must be prepared freshly, since storage results in precipitation. Furthermore, the fabrication of mixed metal aluminum oxides from alkoxides can be problematic, since the alkoxides can have different hydrolysis rates leading to phase segregation in the gels. Also the significant shrinkage of sol-gel based precursors leads to extensive cracking. For this reason fabrication of large ceramic objects via sol-gel routes is not generally feasible. In combination, these issues make sol-gel routes to monolithic aluminum oxides inconvenient for many applications.
It is desirable, therefore to identify materials and processes that can be used to prepare complex metal aluminum oxides starting with stable low cost precursors. The synthetic process should lend itself to the fabrication of a wide range of metal aluminum oxides with fine control over composition and particle size. The present invention provides a method of making metal aluminum oxides that provides these benefits and avoids many of the problems of prior art processes.